Orthogonal Frequency Division Multiplexing (OFDM) is a known method for transmitting digital data, characterized by its sending different bit positions of a serial data stream in parallel, through different sub-channels of a given frequency band. OFDM has advantages over other frequency multiplexing (FDM) schemes, including better spectral efficiency and more robust resistance to channel degradation and narrowband interference.
OFDM has better spectral efficiency because the sub-channels are harmonics of one another, which generally removes the requirement for guard bands between the sub-channels.
OFDM has better resistance to channel degradation and narrowband interference because OFDM transmits different bit positions of the same serial data stream through different frequency bands carrying the sub-channels. Therefore, degradation of just one or two of the sub-channels means that only the bits allocated to those sub-channels are lost. This is significant because, as known in the signal propagation arts, interference and degradation, e.g., from multi-path and from external narrowband sources, is often limited to specific bands. Some FDM schemes, in contrast, may place an entire communication within one band or channel. Channel degradation therefore may result in complete loss of the communication. Further, with OFDM, if error correction is used and the number of sub-channels (i.e., specific bit positions) lost is within the recovery capability of the correction scheme, the original signal may be fully recovered.
Because of such performance advantages, and other advantages such as, for example, ease of sub-channel equalization, OFDM is increasingly seen as a preferred method to transfer digital data.
Various implementations of OFDM systems are known in the art, but most are similar in their general arrangement. A typical OFDM transmitter system receives a serial data stream and partitions the stream into blocks of, for example, M bits. Typically the M bits are encoded as a block of N Quadrature Phase Shift Keying (QPSK) symbols, two bits per symbol, such that N=M/2. QPSK is the most common encoding. Binary phase shift keying (BPSK), encoding one bit per symbol, and X-ary Quadrature Amplitude Modulation (QAM), encoding X bits per symbol, exist but are not generally used—for reasons of practicality and spectral efficiency that are known in the art. Assuming QPSK, there are four possible normalized symbol values, which are (1,1), (−1,1), (−1,−1), (1,−1), representing bit pairs of, for example, (1,1), (0,1), (0,0), (1,0). These representations and mappings of symbol values and bit pairs are only illustrative examples.
As known in the art, each of the N QPSK encoded symbols is transmitted by modulating the respective phases of a respective one of the N different I-Q sub-carriers such that, viewed on an I-Q scatter plot, the phases of that sub-carrier are in one of four quadrants. Typically, all N of the QPSK coded symbols are converted, in parallel, to N corresponding I-Q modulated sub-carriers by inputting the N I-Q symbol values to an N-point Inverse Fast Frequency Transform (IFFT) processor. The N-point IFFT processor outputs N frequency bins, each bin having an in-phase component and a quadrature component. Assuming, for purposes of this disclosure, QPSK, the respective signs of the in-phase and quadrature components are positive or negative, to form the four QPSK states that represent the four two-bit symbols. The N bins output from the N-point IFFT are input to a radio frequency (RF) transmitter that transmits a given band, which will be labeled for reference as “FB”, having N sub-channels, each with a QPSK (or other QAM) signal corresponding to one of the N-point IFFT outputs.
The description above is a simplified overview, omitting discussion of, for example, convolution encoding performed on the original serial data stream prior to the BPSK encoding. These are processes are known, and not pertinent to understanding the present invention.
A typical OFDM receiver is a reverse of the above-described transmitter, and related art FIG. 1 shows an example functional block illustration of a typical conventional digital OFDM receiver 10. Referring to FIG. 1, the example 10 comprises antenna 12, radio frequency (RF) receiver 14, analog-to-digital converter (ADC) 16, N-point digital FFT processor 18, and OFDM decoder/demultiplexer (not shown). The antenna 12 and RF receiver 14 typically receive a frequency band including the band FB transmitted from a sender (not shown). The band FB includes the plurality of N sub-channels, each having in-phase and quadrature components representing, for example, QPSK (or other QAM) symbols encoding bit position(s) allocated to the sub-channel.
Referring to FIG. 1, the example RF receiver 14 includes an in-phase baseband converter 18 and a quadrature baseband converter 20, which convert the frequency band FB to an in-phase baseband signal IB and a quadrature baseband signal QB, respectively. The converters 18 and 20 typically use respective phases of the same synthesized mixing signal (not shown). As known in the art, the mixing signal may be synthesized to be frequency and time synchronized to the received RF signal.
With continuing reference to FIG. 1, the typical RF receiver 14 includes linear automatic gain control circuits 22 and 24 to controllably amplify the in-phase and quadrature baseband IB and QB, respectively, and output corresponding gain-adjusted signals IB′ and QB′.
The FIG. 1 depiction of a typical RF receiver is general; as known to persons skilled in the OFDM and related arts, a typical RF receiver such as item 14 also includes various low pass and bandpass filters (not shown), controllers and circuitry (not shown) for synthesizing the mixing signals feeding the in-phase baseband converter 18 and the quadrature baseband converter 20. These components, and all other design aspects of OFDM front-end RF receivers such as example 14 relating to practicing the present invention are well known to persons skilled in the relevant arts and, therefore, detailed description is omitted.
Referring to FIG. 1, the receiver 14 outputs the gain-adjusted in-phase signal IB′ and quadrature signal QB′, typically on differential signal lines such as lines 30 and 32, respectively. High speed analog-to-digital converter (ADC) 34 samples the gain-adjusted in-phase baseband signal IB′, and high speed ADC 36 samples the gain-adjusted quadrature baseband signal IQ′. The minimum sampling rate of the ADCs 34 and 36, according to the Nyquist Sampling Theorem, must be at least twice the bandwidth of the entire FB band. An example FB bandwidth is approximately 500 MHZ, which requires an ADC sampling rate of at least 1 gigasamples per second (Gs/S).
The required bit resolution of the ADCs 34 and 36 is set by the fidelity requirements, e.g., bit error rate, of the OFDM system. The bit resolution of known implementations of ADCs, particularly the high speed ADCs required to sample the entire FB band also bounds, at least in part, the attainable fidelity or bit error rate of the system. A typical fidelity specification requires an ADC resolution of four to ten bits. The only technology of known, commercially available ADCs that can provide this resolution and sample rate is the technology known in the art as “full flash.” As also known in the art, full flash ADCs typically consume considerable power.
With continuing reference to FIG. 1, ADCs 34 and ADC 36 are typically clocked synchronously and output respective pairs of digital samples. The pairs of digital samples are input to the N-point digital FFT processor 18. The mode of input depends on the configuration of the N-point digital FFT processor 18 but, typically, after a succession of N sampled pairs is input the N-point FFT processors performs digital processing and, after a certain delay, outputs N complex valued results, each having a “real” or in-phase data and an “imaginary” or quadrature data. A post-FFT stage (not shown) decodes each of the N complex valued results into, assuming QPSK, one of four possible symbols—based on the relative sign of the in-phase and quadrature components. The symbol translates to a two bit pair that, assuming no error, is the two bit pair that was encoded by the transmitter as described above.
Various kinds, architectures, technologies and implementations of the N-point digital processor 18 are known. An overriding requirement is that the digital processor must generate N-point FFTs fast enough to keep up with the sample rate of the ADCs. Although an FFT is more efficient than a standard DFT, the computational requirement digital calculation of an FFT is high. Further, OFDM FFTs must be calculated successively, one after the other, at a rate high enough to keep up with the ADC sample rate.
The present inventors have therefore identified at least three shortcomings or problems with digital FFT based OFDM receivers.
The first is the fidelity limit imposed by the bit resolution of high speed flash ADCs. Referring to FIG. 1, the ADCs 34 and 36 sample the entire FB band transmitted by the sender, i.e., all N of the QPSK modulated sub-channels. This band may, for example, be approximately 500 MHz wide. This requires an ADC sample rate of 1 Gs/S—minimum. Flash ADCs are therefore required. Flash ADCs, especially having high sampling rates, are currently limited to the six to ten bit range.
Power is the second problem. The power problem is related, in significant part, to the sample rate of the ADCs. Flash ADCs, necessary because the entire band of QPSK modulated sub-channels must be digitized, consume significant power.
Hardware complexity and chip real estate is the third problem. The high throughput requirement for the digital FFT processor requires significant transistor count and complexity. A high clock rate is also required, which adds to the power problem.